The present invention relates to fluid pressure operated disc brakes for commercial sized automotive vehicles, which incorporate independently operable auxiliary and service brake systems, as well as a parking brake.
Disc brakes, which are today commonly employed in the passenger vehicle field, are finding increasing popularity among commercial sized vehicles, particularly those in the light and medium weight range where purely hydraulic brake systems are employed.
When employing disc brakes on commercial vehicles in the heavy weight range, it is common practice to combine with both the service and parking brake systems an auxiliary brake system capable of producing incremental changes in the braking action.
Typically, such heavy duty disc brakes are pneumatically operted, utilizing a spring-loaded cylinder in combination with the auxiliary diaphragm cylinders mounted outside the conventional wheel brake. Such a cylinder arrangement has come to be known as a "tristop" brake cylinder. While operatively satisfying the braking requirements, such an arrangement requires considerable space, so that only the rear wheels are normally so arranged. Consequently, inadequate parking brake forces are obtained when a loaded vehicle is parked on a grade, for example, while furthermore, the advantages of the auxiliary brake function are only realized at the rear wheels. Moreover, these "tristop" cylinder arrangements are considerably expensive, thus negating their use on all the wheels, even where space conditions otherwise permit such installation.
Another known disc brake arrangement combines the service brake with a spring-loaded parking brake, which is also employed to provide the auxiliary brake. Such an arrangement, however, has the disadvantage of being unreliable, since over the coarse of time the spring becomes fatigued and may ultimately break. Moreover, a further disadvantage of this spring-loaded brake resides in the fact that springs have relatively large tolerances and, as a result, often produce a considerable difference in the various wheel brake forces.